We previously demonstrated that under hyperinsulinemic or hyperglycemic conditions glucose transport is not rate-limiting for glucose disposal in the rat hindlimb. In the present study we evaluated whether a similar limitation of the capacity of muscle to metabolize glucose exists in man. We also measured the relative contributions of increasing glucose and insulin concentrations on glycogen synthase activity, an enzyme potentially important in determining rates of glucose storage into muscle. A total of 88 separate studies were performed in 22 Caucasian males. Glucose uptake rates were measured at 4 different glucose concentrations at 4 insulin levels. At the lowest insulin level, the Michaelis constants (Ks) for glucose disposal in whole body and across the forearm were compatible with a Ks determined in vitro for the transport system. At higher insulin levels, the apparent Ks increased significantly in whole body and across the forearm. We interpret the apparent increase of Ks by insulin to reflect a shift in the rate-limiting step from glucose transport to some step beyond transport. Activation of glycogen synthase by insulin was highly correlated with stimulation of whole body glucose disposal by insulin, especially at high rates of glucose disposal where glucose storage rather than oxidation predominates. Glucose has no effect on glycogen synthase activity. Taken together, these results suggest that post transport processes determine the rate of glucose disposal during insulin stimulation in normal subjects.